D1R1 via Cornice Drop on Corkscrew

Location Name: 
Tallac
Region: 
Desolation Wilderness Area (including Emerald Bay)
Date and time of avalanche (best estimate if unknown): 
Tue, 03/22/2011 - 12:00
Location Map: 
United States
38° 54' 54.54" N, 120° 6' 5.508" W
See map: Google Maps
US


Red Flags: 
Recent avalanche activity
Recent loading by new snow, wind, or rain
Obvious avalanche path
Observation made by: Professional Observer
Avalanche Observations
Avalanche Type: 
dry,slab,
Slope: 
45degrees
Trigger type: 
Other - explain below
Crown Height: 
1 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
60ft.
Terrain: 
Near Treeline
Elevation: 
9 100ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
100ft.
Number of similar avalanches: 
1
More detailed information about the avalanche: 

While ascending; mitt pits, cutting skin track and stepping on winds rolls pointed towards stability. Settlement cones observed on trees all the way to top of Corkscrew.

A ~12hrs old small natural was observed at Corkscrew ridgetop climber's right. En route to look closer, we dropped a small ~200lb piece of cornice with small loose snow results. But a 2nd, much larger ~1000lb cornice drop succeeded in producing a small slab similar in size and depth to the recent natural.

Skiing adjacent to flank produced no signs of instability.

ECT scores were a bit surprising considering the large trigger required. The 18 to 30cm layer had significant riming and small graupel, but graupel was very much the secondary grain type after DFs.

Pic 1: from point of cornice drop.

Pic 2: Just under deepest crown at 30cm, and also just under pit location. Point of cornice drop is looker's right of cornice. What appears to be a step-down is not (likely a creep crack, possibly a recently buried crown - though pit didn't show another potential weak layer).

Snowpit or crown profile photo or graph: 
Avalanche Photos: 
Weather Observations
Blowing Snow: 
Yes
Air temperature: 
Below Freezing
Wind Speed: 
Moderate
Precipitation: 
Snow
Air temperature trend: 
Warming
Wind Direction: 
Southwest
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: 

Very light snowfall under BKN skies which also allowed for strong solar radiation.